The field of the invention is carburetors and the invention relates more particularly to side draft carburetors. Carburetors having a vacuum controlled piston holding a needle have been known for many years and are generally referred to as "SU" carburetors. Such carburetors were typically provided with a fluid damping means to dampen the movement of the piston. They are also provided with a diaphragm held jet assembly and are designed to be used with a fuel pump supplying gasoline under some positive pressure.
The SU design has not proved successful in motorcycles. This is because a motorcycle engine does not provide the amount of engine vacuum to operate the carburetor. Also motorcycle engines do not use a fuel pump and instead rely only on a gravity feed.